The most common cause of blood in the urine is still urinary tract infection. Typically, this is also associated with pain with the passage of blood in the urine.
The other common cause would be urinary tract stones. Stones can be found in the kidney, the ureter, the bladder or even the urethra. These stones can cause bleeding as they move along the urinary tract.
For men, especially more senior individuals, the prostate can be the cause of blood in the urine as enlargement of the prostate can also be associated with the growth of more blood vessels, making it more prone to bleeding.
The most important condition that a urologist would want to rule out would be malignancy or cancer of the urinary tract, most common being bladder cancer.
Passing blood in the urine is NOT normal. A trip down to the urologist's office is definitely warranted.